Spectroscopic Observations of Bright and Dark Emission Features on the Night Side of Venus

Near-infrared spectra of a bright and a dark thermal emission feature on the night side of Venus have been obtained from 2.2 to 2.5 micrometers (μm) at a spectral resolution of 1200 to 1500. Both bright and dark features show numerous weak absorption bands produced by CO$_2$, CO, water vapor, and other gases. The bright feature (hot spot) emits more radiation than the dark feature (cold spot) throughout this spectral region, but the largest contrasts occur between 2.21 and 2.32 μ m, where H$_2$SO$_4$ clouds and a weak CO$_2$ band provide the only known sources of extinction. The contrast decreases by 55 to 65 percent at wavelengths longer than 2.34 μm, where CO, clouds, and water vapor also absorb and scatter upwelling radiation. This contrast reduction may provide direct spectroscopic evidence for horizontal variations in the water vapor concentrations in the Venus atmosphere at levels below the cloud tops.